I. Field of the Invention
The present invention relates generally to electronic measuring systems, and more particularly to an electronic scale employing a variable duty cycle pulsed power supply and which compensates for transducer and component temperature variation with a single temperature sensor.
II. Description of the Prior Art
Electronic systems which measure weight are commonly known. Typically, in an electronic scale, a continuous voltage is applied across a load cell arranged in a wheatstone bridge. The voltage across the load cell output is measured, from which the magnitude of the load applied to the cell is determined.
In a battery operated scale, it is desirable to minimize power consumption to extend battery life. In early electronic scales, the electrical components and the load cell each used comparatively large amounts of power. With the advent of digital circuitry, the power consumption of the logic components began to shrink as compared to the load cell. Thus, systems were developed to reduce load cell power consumption.
Keen et al., U.S. Pat. No. 4,238,784 discloses an electronic measuring system with a constant dual-frequency pulsed power supply. The system pulses the power at the load cell input, thereby utilizing lower power than with a continuous D.C. power supply. The use of pulsed power was also found to reduce the operating temperature of the transducer. This results in a lower steady-state operating temperature, and therefore reduces heat related error and eliminates the need for system warm-up.
Even with pulsed systems, the zero reading of the transducer tends to drift. This effect is ordinarily caused by temperature changes in the system, and by residual voltages in the system during a no-pulse cycle. Various techniques have been applied to compensate for drift, such as disclosed in U.S. Pat. No. 4,238,784 in which drift is compensated for by storing in a capacitor the residual voltages in the electronic components which remain during a no-pulse. This value, representative of system drift, is then subtracted from the sensor output voltage during a pulse. This results in a transducer output compensated for residual error voltages; however, although satisfactory in some circumstances, this system does not enable the pulses to be varied with respect to duty cycle and frequency to thereby further reduce power consumption to lower desired levels.
In order to try to compensate for effects caused by temperature changes, prior art battery operated scales employed separate temperature sensors for the separately housed detection and display circuitry. Such a prior art arrangement has not only resulted in increased cost, but has also resulted in reduction in correction accuracy.
Such prior art electronic scales have been used in the past as checkweight indicators in which the scale is used to determine whether the weight of an object is within a prescribed tolerance (i.e. over a minimum weight, and under a maximum weight), not its precise weight. These prior art scales generally incorporate separate display elements to indicate an over/under condition, usually a series of LED's. Such a prior art configuration again, not only adds cost to the scale by requiring the additional display elements and space on the display face, but the LED's employed also increase the power consumed by the system, and further exacerbate the unwanted problem of heat in the circuitry housing.
These disadvantages of the prior art are overcome by the present invention.